Add a new field to the StackBuilder catalog to specify a manifest URL.

The manifest URL (which is optional) will point to an XML file that
describes the command line, option file and environment options that
can be passed to an installer, to allow automated or non-interactive
deployment tools to figure out what info is required to run an
installer and how to pass it to it.

Requires SQL:

ALTER TABLE downloads_stackbuilderapp ADD COLUMN manifesturl character varying(500);
UPDATE downloads_stackbuilderapp SET manifesturl = '';
ALTER TABLE downloads_stackbuilderapp ALTER COLUMN manifesturl SET NOT NULL;